The plaintiff, Anthony Michaels of San Diego county, alleges that he signed up for a free membership to the site last Christmas Eve, but then upgraded to a paid one after receiving e-mail ads stating that other schoolmates were trying to contact him. Those statements turned out to be false, according to the lawsuit.

Michaels' lawyer, Brian Kabateck, said his client had no way of verifying whether his former schoolmates were actually seeking to contact him on the site, short of signing up for a one-year membership.

"The e-mail said: 'So and so's trying to find you, and in order to hook up with him you have to join and become a gold member,'" Kabateck said. "It's their business model: Once you're in for free, they try to upsell you as quickly as possible," he added.

A spokesman for Classmates.com said the company had no comment on the lawsuit.

Michaels, who is seeking class-action status, alleges misrepresentation, negligence, fraudulent concealment and violations of California's business code. He brought the case against Classmates Online, Classmates Media Corporation and parent company United Online, based in Woodland Hills, Calif. in state court.

Kabateck, who is probably best-known for successfully suing Google for click fraud, said he intended to ask for refunds for all Classmates.com users who joined the site in response to a misleading email.

Internet legal expert Venkat Balasubramani said that California's business code offers Michaels powerful ammunition against Classmates.com. "If you do anything unfair, improper or that violates a regulation, you could be committing unfair competition," Balasubramani said.

This case is not the only potential class-action lawsuit against an alumni networking site. Reunion.com also faces a lawsuit by members complaining about the site's marketing efforts. In that case, filed in federal district court in San Francisco, the members allege that Reunion.com sent e-mails that appeared to have come from specific friends, but were actually sent by the site.

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